1. Dry the mushrooms and slice.
2. Cook the onion and butter or margarine in a pan for 2 minutes, add the mushrooms, cover and cook gently for 5 minutes.
3. Sprinkle in the flour and seasoning and cook for 2 minutes.
4. Remove pan from the heat and stir in the remaining ingredients.
5. Lightly butter an ovenproof dish and put in the mushroom mixture.
6. Bake in a moderate oven (180°C, 350°F, Gas Mark 4) for 15 minutes.
7. Serve with noodles and green salad.